A short sea trip later, retracing our previous path, we pull into the bay again at Port Electron. Despite the lessons from the garrulous pilot, our brief visit to Atom Land has shown us that we need more answers from here before heading into the interior. We disembark, hoping to avoid meeting the same pilot, and spread out to explore. This is what we find out.
The electron was the first subatomic particle to be discovered. These tiny objects were first observed as beams of so-called “cathode rays,” a strange radiation emitted by metals when they are heated. Some thought the rays were made up of particles, while others thought they were waves in the ether. Two decades after their initial discovery, J. J. Thomson, working in Cambridge in the UK in 1897, appeared to settle the matter in favor of particles.
Particles have a definite mass and a definite electric charge, which also means that the ratio of the mass and the charge has a definite value. To prove that cathode rays were made of particles, one thing Thomson needed to do was to show that this ratio was always the same, regardless of what material was used as the source of the cathode ray. This would meet the criteria for calling them “particles.”
The first key evidence is the fact that cathode rays are deflected in electric and magnetic fields in just the way that would be expected for a beam of charged particles. No wave known at the time carried charge, so this might be considered strong circumstantial evidence in favor of the particle hypothesis.
The second piece of evidence Thomson acquired came from finely balancing the electric and magnetic fields he applied to a beam of cathode rays as it traveled through a vacuum. He was able to arrange it so that the forces from each field canceled each other out completely, so there was no net force. From this setup, the speed of the beam can be worked out.10
Finally, once the speed is known, the magnetic field can be turned off, and the amount the beam deflects in the electric field allows the ratio of the charge and the mass to be determined.11 As Thomson observed, the ratio for the electron is about two thousand times higher than it is for the hydrogen ion, which is a single proton—the lightest particle known at the time. This meant that either an electron carried enormously more charge than a proton, or it had much less mass.
There are lots of ways of working out which of these options is true. Probably the most annoying way, which the pilot would doubtless be eager to demonstrate if we hadn’t managed to avoid him this time, is to suspend small charged spheres in an electric field. The electrostatic force on the sphere depends both on the strength of the electric field and on the electric charge that happens to be on the sphere. If the sphere is stationary—not falling or rising—this force must be exactly canceling out gravity, which depends on the mass of the sphere. So if the field strength and the mass of the sphere are known, the charge can be calculated. Doing this many times shows that the charge is always a multiple of a small unit, which we call e. Spheres can carry a charge of e, or two or three times e, or a hundred times e, but never a half e, or any fraction. This e is the charge of the electron.12
The result of all this evidence is that there is a tiny particle, the electron, with a definite mass and definite charge. Since electrons are so much smaller than atoms, it is a sensible guess to assume they are present somewhere inside atoms, before being split out to form cathode rays.
At this point we are truly ready to return once more to Atom Land and begin the exploration of the interior.